Chemical-mechanical polishing (CMP) is a widely used means of planarizing silicon dioxide as well as other types of surfaces on semiconductor wafers. Chemical mechanical polishing typically utilizes an abrasive slurry disbursed in an alkaline or acidic solution to planarize the surface of the wafer through a combination of mechanical and chemical action.
FIG. 1 illustrates one type of chemical mechanical polishing (CMP) system. The CMP system 100 includes a rotatable circular platen or table 102 on which a polishing pad 104 is mounted. A single or multi-head polishing device 106 is positioned above the table 102. The polishing device 106 has a single or multiple rotating carrier heads 108 to which wafers can be secured typically through the use of vacuum pressure. Typically, the polishing pad 104 includes a bottom pad 110 mounted on the platen 102 and a top pad 112 mounted on the bottom pad 110. Typically, the top pad 112 is adhered to the bottom pad 110 using a glue. The bottom pad 110 serves as a damper and typically is formed from foam or felt. The top pad 112 generally contacts the wafer for polishing and is typically formed from polyurethane.
In use, the platen 102 is rotated and an abrasive slurry is disbursed onto the polishing pad 104 of the platen 102. Once the slurry has been applied to the polishing pad 104, the rotating carrier heads 108 move downward to press their corresponding wafers against the polishing pad 104. As the wafer is pressed against the polishing pad 104, the surface of the wafer is mechanically and chemically polished. Between polishing runs, the polishing pad 104 is typically conditioned. Conditioning typically includes applying a conditioning tool, such as a diamond impregnated steel plate, to the top pad 112 to remove expired surface and expose fresh pad material.
A significant goal relating to chemical-mechanical polishing techniques is the maintenance of substantially uniform removal rate over the entire surface of a given wafer. The uniformity or nonuniformity of a wafer is typically measured using the relationship: .sigma./R, where R is the average removal amount over a number of different locations on a wafer and .sigma. is the standard deviation of the removal amounts. The polishing uniformity of a polishing pad may also be measured using the relationship .sigma./R for wafers polished by the pad over time. By way of example, FIG. 2 is a graph illustrating polishing uniformity as a function of pad life for a typical polishing pad. As can be seen, the polishing uniformity typically starts poorly in a period of time known as the break-in period. This typically results from the pad polishing the center of a wafer slower or faster than the edges. After the break-in period, the polishing uniformity reaches an optimum level and flattens out for a period of time. This time period is commonly referred to as the useful life of the pad. At the end of the useful life, the polishing uniformity declines, again usually resulting from the pad polishing wafer centers faster or slower than the edges.